1. Field of the Invention
The present invention relates to the field of electronic isolators.
2. Background Art
Electronic circuits are often made up of a number of discrete “stages” where the output of a first stage is provided as input to a subsequent second stage. To maximize performance, it is sometimes desired to provide isolation between the stages, so that the operation of the first stage is not affected by the operation of the second stage. Generally, this has been accomplished by employing an isolator between the circuit stages. However, currently available isolators suffer from a number of disadvantages.
Referring to FIG. 1, a multistage circuit is symbolically indicated. The circuit consists of a source stage 100 that provides output to a load stage 120. The source and load stages are separated (and isolated) by an isolator 110. The isolator 110 minimizes unwanted interactions between the source and load stage and protects circuitry of the source stage from damage due to power reflections from the load stage. Currently isolators are implemented by one of several methods, including magnetic isolators and circulators, rat race circuitry, and resistive pads. These solutions all have significant limitations in performance and applicability.
Magnetic Isolators
The magnetic isolator is a common implementation of the isolator function. Magnetic isolator devices use Faraday rotation to differentiate between waves traveling in different directions. A magnetic isolator generally consists of a magnetic material (typically a ferrite) sandwiched between two poles of a permanent magnet. The effective operation of such a device requires that a significant portion of a wavelength be physically present in the device. This requirement in turn effectively determines the physical size of the magnetic isolator. (The size is also dependent on both the permitivity and permeability of the ferrite material as well as the strength of the permanent magnet that is used to bias the ferrite material.)
The magnetic bias is used to create a non-reciprocal environment within the ferrite that induces a polarization change in a propagating wave. This polarization change is used to direct the wave along a particular path, either to the output or to a dummy load. Since these devices make use of the wave characteristics of the signal, changing the frequency changes the dependent phase characteristics, and the resulting isolation parameters of the isolator. The result is a relatively narrow band device (10%-20% being typical) that is relatively large and costly. As a result, the use of magnetic isolators is typically restricted to certain applications where the size can be accommodated, such as test equipment and the interface between high power amplifiers and antennas being the principle applications.
Rat Race Circuit
The rat race circuit is a closed loop or circuit path. It relies on the constructive and destructive interference of signals to produce low insertion loss and high isolation effects. If a transient signal is introduced at a point along the closed path, it propagates in both directions. At some point(s) the two signals constructively interfere and an output port of the proper characteristic impedance can be located there. The signals destructively interfere at the input port as they continue to propagate around the circuit or are reflected off the output port. Similarly, it is possible to locate a point(s) where a signal injected at the output port constructively interferes while at the same time the same signal injected at the input destructively interferes. This point is the location of the third port in a circulator configuration.
A disadvantage of the rat race approach is that as an interference system, it is inherently narrow band. The second problem is that the path length of the closed loop must be a significant portion of a wavelength. Preferably, it will be several wavelengths long. Thus, even at microwave frequencies, the path may be too long to be practical. It is certainly much too long to be incorporated into an integrated circuit much below W-band.
Pads
Another approach has been the use of a “pads” which are resistor networks having specified input and output characteristic resistances and a specified attenuation of the input signal. Pads operate by attenuating the signals for both forward and reverse directions. This isolation reduces the amount of feedback to the source stage but requires the source stage to provide a higher gain and output power to compensate for the loss in the desired input to the load stage. For example, if it is desired to have 10 dB of isolation between stages of an amplifier, then the pad would need to be a 10 dB attenuator and the previous stage would need to have 10 times the gain and 10 times the output power. This additional power requirement may be too expensive to achieve or may unduly limit circuit performance.
Pads can be made asymmetric by having different input and output resistances. However, these are typically used where a true impedance transformation is desired. In most applications, the impedance levels are the same and standardized to enable easy interface with commercial test equipment. As a result, the use of isolation pads is usually limited to low power circuits and compensation for the loss is achieved by added gain stages, or added gain and power from the down stream circuitry, an expensive and complicated solution.